1. Field of the Invention
The present invention relates to a current detection circuit for detecting current that flows through a photo detector device.
2. Description of the Related Art
Various kinds of electronic devices provide a function of measurement of light input externally, and a function of performing signal processing according to the received light amount thus measured. Examples of such electronic devices include a luminescence sensor, a photo detector device employed in an infrared ray remote controller, and so forth. Examples of photo detector devices for receiving light which are widely used include a phototransistor, a photodiode, a CCD (Charge Coupled Device), etc.
Each of the phototransistor and the photodiode outputs current according to the light thus received. Accordingly, the photo detector device amplifies the current that flows through such a photo detector device, or performs signal processing such as voltage conversion. For example, Patent document 1 discloses a configuration of a photoreceptor circuit employing a computation amplifier.
[Patent Document 1]
Japanese Patent Application Laid-open No. 2005-216984
As described in Patent document 1, in general, conventional photo detector devices have a configuration in which a resistor is serially connected to a photo detector device such as a photodiode, a phototransistor, or the like. With such conventional arrangements, signal processing is performed for the voltage at the resistor. However, such a circuit configuration has a program in which, in a case that the photocurrent that flows through the photo detector device changes, the voltage drop across the resistor changes. This leads to a change in the electric potential at the node between the photo detector device and the current detection circuit, i.e., at the cathode of the photodiode or at the collector of the phototransistor. This leads to fluctuation in the bias state of the photo detector device, resulting in a problem that the current value of the photocurrent fluctuates even in the state in which a constant light amount is input.
Also, let us consider a case in which the photocurrent increases, which leads to increased voltage drop across the resistor. In this case, the voltage is reduced between the collector and the emitter of the phototransistor, leading to a situation in which the phototransistor cannot operate. That is to say, in some cases, such a conventional arrangement limits the dynamic range.